Current wireless communication systems provide the ability for users to communicate to and from wireless or mobile users. There are generally two types of wireless communication systems, circuit-switched (“CS”) and packet-switched (“PS”).
In typical circuit-switched wireless communication systems, the mobile switching center (“MSC”) connects the landline public switched telephone network (“PSTN”) system to the wireless communication system. The mobile switching center is typically split into an mobile switching center server and a media gateway (“MGW”), and incorporates the bearer independent call control (“BICC”) or the integrated services digital network user part (“ISUP”) call control protocol for call delivery between mobile switching centers.
The current approach to introducing internet protocol (“IP”) multimedia services for universal mobile telecommunications service (“UMTS”) and code division multiple access (“CDMA”) third generation (“3G”) systems is to define a brand new internet protocol multimedia subsystem (“IMS”), comprised of a set of internet protocol connected network entities within the internet protocol multimedia subsystem using packet-switched services. These network entities provide internet protocol multimedia features and services using the session initiation protocol (“SIP”) as the primary vehicle for call control.
The legacy circuit-switched communication system works with the internet protocol multimedia system to provide a more convenient upgrade path from the legacy circuit-switched communication system. The combined system provides packet-switched functionality to an existing circuit-switched communication system.
The gradual process of upgrading legacy circuit-switched communication systems to packet-switched communication systems creates a communication system where legacy circuit-switched portions and packet-switched portions co-exist. As a user of a mobile telephone roams between legacy circuit-switched communication systems and packet-switched communication systems, handoffs must occur to maintain an active call.
Thus, a need exists for a communication system with both packet-switched portions and legacy circuit-switched portions that can hand off an active call between the packet-switched portions and the legacy circuit-switched portions.